A severed finger sparked an online uproar that went viral. And very quickly, rattled authorities here took note.

The story of Sun Zhongjie, a 19-year-old driver who chopped off his finger to decry police entrapment, shows how the Internet has become an effective tool of public protest in this tightly controlled country.

Almost every form of open dissent is outlawed in China, but mass protests organized online are increasingly putting pressure on police, judges and other officials -- and getting results.

Last June in Hubei province, an online campaign by netizens, as they are popularly called here, helped free a 22-year-old waitress arrested for killing a local official in what appeared to be a clear case of self-defense. In Nanjing, a top official was expelled from the Communist Party and jailed after angry netizens posted photos online of him smoking expensive cigarettes, sporting a pricey watch and driving a Cadillac.

Across the country, online petition drives and surveys have prompted police to reopen closed cases, authorities to cancel unpopular development projects and the party's national leadership to fire corrupt local officials.

In the view of academic experts, lawyers, bloggers and others here, the Internet is introducing a new measure of public accountability and civic action into China's closed and opaque political system.

"This is the era of disguised accountability," said Hu Xingdou, a sociology professor at the Beijing Institute of Technology. "That means holding government officials accountable by relying on the Internet rather than on traditional means like elections and the checks by the Congress."

For the moment, the central government in Beijing appears to be allowing Internet protests to continue, and in some instances even encouraging them -- as long as the campaigns are confined to local issues and target local officials. It appears to be a way for the central government to keep track of what is happening in the provinces while demonstrating that it is responsive to citizens' concerns.

"Now senior officials go to the Internet to find out what crimes are being committed by local officials," said Mo Shaoping, a prominent Beijing lawyer who specializes in human rights and press freedom. "In some cases, the government wants to know the public mood, or they want to punish some local official who isn't following policies."

"This Internet power has a huge influence on the government," Mo said. "But it's hard to tell if they are worried."

Plenty of control

The Internet is still subject to tight government control in China. Many foreign Web sites are blocked, including human rights sites, news sites such as BBC, and popular U.S.-based social networking sites. YouTube is also blocked.

Instead, China has its own cloned local versions of Facebook, Twitter and YouTube that are more easily monitored and subject to government restrictions.

Internet cafes are strictly regulated, with users required to leave their identification numbers, and cafe owners made responsible. Chinese who post comments or blog items critical of the Communist Party are routinely tracked down and arrested. News sites and discussions that become too critical are shut down.

But the Internet is booming nonetheless. By some official statistics, China had as many as 350 million Internet users by the first half of this year. More than half -- some 182 million people -- have their own blogs.

As usage grows, netizens have found more sophisticated ways to get around the restrictions. And the number of viral protests keeps increasing.

Legal victory

Sun became a hero of sorts here in Shanghai for his protest against police entrapment.

He was driving his company's minivan on an errand last month when a man flagged him down and begged for a lift. A few minutes later, policemen surrounded Sun's vehicle and accused him of operating an illegal taxi. The van was confiscated, Sun was fined 10,000 yuan, or about $1,400, and his company fired him.

Drivers in Shanghai had been complaining for years about such sting operations. In most cases, drivers angrily pay the fines, which they consider a form of extortion.

But Sun decided to fight back. He chopped off the pinky finger on his left hand as a public way to declare his innocence. Soon, his story was picked up in several national newspapers. The story then spread online, with unregulated Internet bulletin boards, chat rooms and the popular instant messaging site QQ inundated with complaints of police harassment and support for Sun.

The uproar forced Shanghai law enforcement officials to investigate. A few days later, they released the results of their probe, saying the police had done nothing illegal.

The Internet protesters erupted. One online site, xinmin.cn, conducted an online poll of 20,000 users, and nearly 98 percent said they did not believe the government's investigation was fair.

On Tianya, a popular bullet board site, one commenter wrote: "Why we pay attention to this case? Because there is a law enforcement agency which robs people brazenly and another investigating agency which labels itself fair but lies through its teeth. . . . It is the sorrow of all people to live in such an era."

The Internet furor was so intense that the local government announced a new investigation. Sun won his case and did not have to pay the fine.

"By just clicking a mouse, people now have the right to check up on the government," said Hao Jinsong, Sun's lawyer.

Hundreds of drivers ensnared over the years showed up this month at the official government complaints office seeking refunds for their fines.

"With just one person's efforts, they wouldn't change," said Liang Wenhua, who was fined after taking the bait and giving a police-paid hitchhiker a lift on the back of his motorcycle. "But with the power of the Internet, things will be different."

Researchers Zhang Jie and Wang Juan contributed to this report from Beijing and Shanghai.